The invention relates to the purification of nucleosides, and more particularly to a selective solvent extraction method for purifying protected nucleosides.
Nucleosides are compounds of importance in physiological and medical research, obtained during partial decomposition, i.e., hydrolysis, of nucleic acids, and containing a purine or pyrimidine base linked to either D-ribose (forming ribonucleosides) or D-deoxyribose (forming deoxyribonucleosides). They are nucleotides minus the phosphate group. Well-known nucleosides include adenosine, cytidine, guanosine and thymidine. Nucleosides are multi-functional compounds, having both amino and alcohol functional groups. In order to conduct syntheses selectively and efficiently, it is necessary to block specific functional groups in order to achieve reaction at the desired sites. The xe2x80x9cprotectingxe2x80x9d groups are designed to be removed under specific carefully controlled conditions, usually under relatively mild and typically acidic conditions. To be useful as precursors in the synthesis of high value pharmaceuticals, it is necessary that protected nucleosides be of very high purity (i.e., greater than about 99% by weight (wt %), preferably greater than about 99.5 wt %). The very sensitive nature of the protecting groups together with the variety of polar and non-polar impurities generated during the syntheses of these derivatives makes their purifications complicated, expensive, and difficult to scale-up to industrial scale production.
Typically, the protection of nucleosides involves the derivatization of both amino and alcohol functional groups. An exception is thymidine, which requires only the protection of alcohol groups. Various schemes are employed to achieve these protected nucleosides, but usually the N-protected derivatives (most often N-acylated) are isolated and purified before protecting the alcohol groups. The presence of the free alcohol groups often leaves these derivatives with sufficient polarity that they can be readily purified by recrystallization. However, when these alcohol groups are also derivatized (most often as trityl ethers), the fully protected nucleosides are usually very difficult to crystallize. Purification then is typically achieved by resorting to column chromatography, usually followed by precipitation of the appropriate column fractions into non-solvents to remove traces of co-eluted colored by-products.
Discussions of the synthesis and protection of nucleosides by derivatization may be found in many references, including the following, all of which are incorporated herein by reference. One method of protecting nucleosides is described in Ti, et al., xe2x80x9cTransient Protection: Efficient One-flask Syntheses of Protected Deoxynucleosides,xe2x80x9d J. Am. Chem. Soc., Vol. 104, 1316-1319 (1982), which is discussed in more detail below in regard to the examples. Other methods of synthesizing protected nucleosides are set forth in Charubala, et al., xe2x80x9cNucleotides XXIII: Synthesis of Protected 2xe2x80x2-Deoxyribonucleoside-3xe2x80x2-phosphotriesters Containing the p-Nitrophenylethyl Phosphate Blocking Group,xe2x80x9d Synthesis 965,(1984). Still other methods for synthesizing such protected nucleosides are set forth in Kierzek, xe2x80x9cThe Synthesis of 5xe2x80x2-O-dimethoxytrityl-N-acyl-2xe2x80x2-deoxynucleosides, Improved xe2x80x98Transient 155- Protectionxe2x80x99 Approach,xe2x80x9d Nucleosides and Nucleotides, 4(5), 641-649 (1985). In all of these references, protection by N-acylation is effected with benzoyl chloride on adenosine and cytidine derivatives, and with isobutyric anhydride on guanosine derivatives, as is wellknown in the art. The compounds are then further protected by the introduction of methoxytrityl or dimethoxytrityl groups, also as is well-known in the art. An earlier article on the protection of such nucleosides may be found in Schaller, et al., J. Amer. Chem. Soc., Vol. 85, 3821-3827 (1963). Another article on protected nucleosides is McGee, et al., xe2x80x9cA Simple High Yield Synthesis of N2-(2-Methylpropanoyl)-2xe2x80x2-deoxyguanosine,xe2x80x9d Synthesis, 540 (1983). In all of the reported syntheses, the protected nucleosides must be subjected to purification prior to their use in pharmaceutical syntheses.
The impurities generated during the various syntheses of protected nucleosides include polar compounds, such as isobutyric acid and benzamide, and non-polar compounds such as dimethoxytrityl methyl ether and the 3xe2x80x2,5xe2x80x2-bis-dimethoxytrityl ether nucleoside derivatives.
On a laboratory scale, recrystallization is widely practiced as a purification method. However, because of the broad range of polarity exhibited by these impurities, purification of protected nucleosides with a single recrystallization solvent system is difficult to achieve. Multiple recrystallizations are often required to achieve required purity levels. As an industrial process, losses (often greater than 10%) of valuable product to the recrystallization medium, and long processing times for mixing, heating, cooling and filtration make this method less attractive.
Column chromatography, especially flash silica gel chromatography, has been used extensively to purify protected nucleosides on a small scale. This method requires the use of large volumes of high purity solvents in proportion to the amount of material purified. The method is also labor-intensive, requiring precise monitoring to make the fraction cuts at the appropriate times to maximize yield of desired product. For these reasons, large-scale use of this method of purification can be very costly.
The equipment required to conduct flash silica gel chromatography on a multi-kilogram scale is expensive to purchase and operate. For example, one commercially available production scale chromatography unit is capable of separating up to about 4 kg of material per run. Run times can vary from 18 to 36 minutes, at an elution rate of 7 liters per minute. The basic unit investment is very expensive, coupled with the cost (and subsequent disposal cost) of 125 to 250 liters of expensive high purity solvent per run. These costs make purification by chromatography unattractive on an industrial scale.
In the purification process of the present invention, solid particles of protected nucleosides are selectively washed to remove undesirable polar and/or non-polar impurities, while leaving the solid nucleoside particles substantially undissolved. In a preferred embodiment of the invention, the process comprises two slurry washing steps. In the first slurry washing step, the particles of protected nucleoside are slurried with a solvent for the polar impurities, in which solvent the particles are insoluble or, at most, only slightly soluble. The solid particles are then recovered from the slurry, as by filtering. In the second slurry washing step, the particles are slurried with a solvent for the non-polar impurities, in which solvent the particles are insoluble or, at most, only slightly soluble. The solid particles are then recovered from the slurry, as by filtering. If desired, one or both of the slurry washing steps may be repeated to remove additional impurities. However, preferably each washing is performed only once, to minimize loss of solids. For purposes of this application, particles of protected nucleoside will be considered to be slightly soluble in a solvent when less than about 10 wt % of the total particles dissolve during a washing step, and preferably less than about 5 wt % dissolve, to minimize loss.
While not wishing to be bound to a particular theory of how this process works, it is believed that when the solvent wets and slightly dissolves the surface of the particles, impurities which may be tied up or attached in any manner thereto are released into solution.
Therefore, although it is desirable to minimize the loss of solids, it is believed that improved washing is obtained when at least about 0.1 wt % of the solid particles of protected nucleoside dissolve in the solvent during a washing step, preferably at least about 0.5 wt %.
The washing steps may be done in any order. However, because the polar solvents tend to be less volatile than the non-polar solvents, it is preferred to follow a polar wash with a non-polar wash, in order to minimize the amount of drying needed to remove excess solvent. This is of particular importance when the polar solvent is water-based, because removing water may require extended drying which could affect the nucleoside particles. If only a single washing step is being done, then preferably a non-water based solvent is used.
In another embodiment of the present invention, a single solvent for both the polar and non-polar impurities is used, in which solvent the particles are insoluble or, at most, only slightly soluble. As in the above process, the solid particles of protected nucleoside are then recovered from the slurry, as by filtering. In this case, only one washing step is required, although it may be repeated if necessary or desired. To avoid the problem of removing water by drying, it is preferred that the solvent in a single step washing process contain as little water as possible, preferably none.
As discussed above, protected nucleosides are multi-functional compounds, having both polar and non-polar functionalities, although generally both functionalities are relatively weak. As a result, the protected nucleosides are soluble in some polar and non-polar solvents, but insoluble in others. Some of the impurities which need to be removed are soluble in solvents in which the protected nucleosides are not soluble. It is simple to remove such impurities by slurry washing, because they can be solubilized without dissolving the protected nucleoside particles. On the other hand, some of the impurities, particularly intermediates or partially reacted nucleoside materials, are only soluble in the same solvents, either polar or non-polar, in which the protected nucleosides are soluble. To dissolve these materials it is necessary to use a solvent which can also dissolve the protected nucleoside particles. The present inventors have found that by mixing a solvent in which a protected nucleoside is soluble with a solvent in which it is insoluble, one can obtain a slurry washing solvent which dissolves the impurities which need to be removed while only dissolving a small amount of the protected nucleoside.
Therefore, in accordance with one embodiment of the present invention, polar impurities are removed from particles of a protected nucleoside by using a mixture of miscible polar solvents, wherein the protected nucleoside is soluble in one of the solvents and substantially insoluble in the other. The solvents need to be ones which otherwise do not affect or react with the protected nucleosides.
A particularly preferred combination of polar solvents is water and acetonitrile.
While protected nucleosides are relatively insoluble in water, particularly cold water, they are highly soluble in acetonitrile. The most effective ratio of acetonitrile to water to use for the purification of a specific protected nucleoside will be a function of the protecting groups used. The optimum ratio is readily determined by a series of experiments in which very small amounts of crude product are slurried with small volumes of solvents, then the filtrates analyzed by HPLC analysis. Any ratio of acetonitrile and water may be used, provided the desired slurry washing is obtained. Preferably the ratio of acetonitrile to water is within the range of about 5:95 to about 30:70, by volume, more preferably from about 10:90 to about 25:75. Such mixtures of acetonitrile in water will effect substantial removal of the polar impurities while removing very little of the protected nucleoside. Other polar solvents which can be used in combination with water include lower molecular weight, i.e., C1 to C6, linear and branched alcohols, including methanol, ethanol, isopropanol, propanol and butanol; dipolar aprotic solvents such as dimethylformamide (DMF), N-methylpyrrolidone, and dimethyl sulfoxide (DMSO); and acetone.
In like manner, non-polar impurities can be removed from particles of a protected nucleoside by using a mixture of miscible non-polar solvents, wherein the protected nucleoside particles are soluble in one of the solvents, and substantially insoluble in the other. As with the polar solvents, the non-polar solvents need to be ones which otherwise do not affect or react with the protected nucleosides.
In accordance with a preferred embodiment of the present invention, the non-polar impurities can be removed with mixtures of hexane and methylene chloride. The protected nucleosides are effectively insoluble in hexane, but they are highly soluble in methylene chloride. A suitable ratio of hexane to methylene chloride to use for the purification of a specific protected nucleoside will be a function of the particular nucleoside and the protecting groups used, as well as the nature of the impurities present. This can readily be determined by one skilled in the art by a series of small scale experiments as described above for the removal of polar by-products. Any ratio of hexane to methylene chloride may be used, provided the desired slurry washing is obtained. However, preferably, the ratio of hexane to methylene chloride is within the range of about 3:1 to about 1:3, by volume, and more preferably within the range of about 3:1 to about 1:1. Typically, a volume ratio of about two parts hexane to one part methylene chloride will effect the removal of the non-polar by-products while removing very little of the protected nucleoside. Cyclohexane may be used instead of, or in combination with, hexane in these formulations. Non-polar solvents in which protected nucleosides generally are insoluble include C5 to C10 linear, branched and cyclic hydrocarbons. Non-polar solvents in which protected nucleosides generally are soluble include ethyl acetate, and chlorinated solvents such as methylene chloride, chloroform, carbon tetrachloride and dichloroethane.
In accordance with another embodiment of the present invention, both polar and non-polar impurities may be removed in a single step by using a mixture of polar and non-polar solvents. The solvent mix must be one in which the product particles are insoluble or, at most only slightly soluble, and the polar and non-polar impurities are soluble. Generally, this is achieved by using a mixture of solvents which are miscible together and capable of dissolving the polar and/or non-polar impurities which are present. Preferably the mixture contains at least one polar and one non-polar solvent. If needed, a third solvent may be used which is miscible with both of the other solvents, to form a three component mixture. This third solvent can be a bifunctional solvent which is miscible with both the polar and the non-polar solvents. It is preferred to avoid using water in such a mixture, to reduce the amount of drying needed after filtration. It is preferred to use an alcohol as the polar solvent in a single step process, with methanol particularly preferred. A suitable mixture of solvents is methanol, in combination with the non-polar solvent mixture of hexane and methylene chloride. Good results were obtained using a mixture of about 80% hexane, with about 10% methanol and about 10% methylene chloride. The mixture of solvents can then be adjusted to maximize the removal of the particular impurities and minimize the loss of nucleoside, as discussed above.
The purification process of the present invention may be used to purify any protected nucleoside. It is particularly useful for purifying nucleosides which are difficult to purify by recrystallization. As discussed above, nucleosides which are protected by derivatization of both amino and alcohol functional groups may lack sufficient polarity to be readily purified by recrystallization. The process is particularly suitable for purifying protected nucleosides in which the alcohol group has been derivatized as a trityl ether, and the amino group has been derivatized as a benzoyl or isobutyryl group. Such nucleosides include adenosine, cytidine and guanosine, and has been found particularly useful for adenosine and guanosine. Thyrnidine is generally protected only by derivatization of its alcohol groups and, therefore, still has sufficient polarity to be purified by recrystallization. However, the slurry washing process of the present invention is also suitable for the purification of thymidine. It is believed that the slurry washing process is simpler, uses less solvent, and retains more nucleoside than recrystallization. One skilled in the art can readily determine whether the slurry washing purification process of the present invention is suitable for use in purifying other particular protected nucleosides.
In the following examples, protected deoxynucleosides are synthesized according to the procedures described by Ti, et al., xe2x80x9cTransient Protection: Efficient One-flask Syntheses of Protected Deoxynucleosides,xe2x80x9d J. Am. Chem. Soc., Vol. 104, 1316-1319 (1982), incorporated herein by reference. The reference describes an application of the concept of transient protection to the synthesis of protected deoxynucleosides. The deoxynucleosides are first treated with trimethylchlorosilane in pyridine for protection of the hydroxyl groups. Then the product is immediately reacted with an acylating group, benzoyl. chloride for deoxyadenosine and deoxycytidine, and isobutyric anhydride for deoxyguanosine, to effect N-acylation. Hydrolysis of the trimethylsilyl groups takes a few hours in aqueous pyridine or a few minutes with dilute ammonia. The ammonia also effects selective hydrolysis of the initially formed N,N-dibenzoyldeoxyadenosine derivative to the desired N-benzoyldeoxyadenosine. This one-flask procedure is described as giving crystalline N-acyl deoxynucleosides of deoxyadenosine and deoxycytidine in 95% yield and deoxyguanosine in 75% yield, in only a few hours. The 5xe2x80x2-O-dimethoxytrityl deoxynucleosides are also obtained in the one-flask procedure by initial reaction of the deoxynucleosides with 4,4xe2x80x2-dimethoxytrityl chloride, followed by treatment with trimethylchlorosilane and then reaction with the acylating group, benzoyl chloride. After simple purification by flash chromatography, the deoxyadenosine and deoxycytidine products are each obtained in 80-90% yield. This same process can be used to form the 5xe2x80x2-O-dimethoxytrityl deoxyguanoside, using isobutyric anhydride as the acylating group.